Method of Communicating which Channel is to be Monitored by Subscriber Units that are Idle in a Communication System

ABSTRACT

The present invention discloses, in a two-way radio frequency (RF) communications system having at least one repeater, a plurality of channels, and a plurality of subscriber units, a repeater receives a message that identifies a channel that is selected to serve as a rest channel for the system. The channel that is currently serving as the rest channel for the system is monitored by subscriber units that are idle in the system. If the repeater determines that the channel selected to serve as the rest channel is hosted by the repeater, the repeater periodically transmits an identity of the channel as the channel currently serving as the rest channel on at least the channel.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is commonly owned by Motorola, Inc. and concurrentlyfiled with the following U.S. Patent Applications:

Ser. No. ______, titled “Method for Trunking Radio Frequency Resources,”which is incorporated herein by reference in its entirety (attorneydocket number CM11832NBH);

Ser. No. ______, titled “Method of Efficiently Synchronizing to aDesired Timeslot in a Time Division Multiple Access CommunicationSystem,” which is incorporated herein by reference in its entirety(attorney docket number CM11833NBH);

Ser. No. ______, titled “Method for Selecting a Channel to be Monitoredby Subscriber Units that are Idle in a Communication System,” which isincorporated herein by reference in its entirety (attorney docket numberCM12118); and

Ser. No. ______, titled “Method for Ending a Call Session in aCommunications System,” which is incorporated herein by reference in itsentirety (attorney docket number CM12112).

FIELD OF THE DISCLOSURE

The present invention relates generally to two-way wireless trunkedcommunication systems.

BACKGROUND OF THE DISCLOSURE

Many varieties of trunked two-way radio communications systems are knownin the art. FIG. 1 is a block diagram illustrating both a typicalconventional radio system 101 and a trunked radio system 103. In theconventional radio system 101, a plurality of subscriber units areformed into talkgroups. Each talkgroup uses separate channels forcommunication. Thus, each talkgroup is served by one channel. Incontrast, the trunked radio system 103 and its subscriber units use apool of channels for virtually an unlimited number of talkgroups. Thus,all talkgroups are served by all channels. The trunked radio system 103works to take advantage of the probability that not all talkgroups needa channel for communication at the same time. Estimates are made abouthow much load a typical user presents to the system in terms of callsper hour and duration of each call. For a traffic load, fewer channelsare required since all talkgroups are served by all channels. Combiningthis with the number of users on the system, and the acceptable qualityof service (QoS), determines how many trunked channels are required tosatisfactorily serve the number of users. With a given number ofchannels, a much greater number of talkgroups can be accommodated ascompared with conventional radio systems. Hence, a primary purpose of atrunked radio system is the efficient utilization of channels allowingfor more users to carry many conversations over a fewer number ofdistinct channels.

As seen in FIG. 2, a trunked radio system can be either a centralizedtrunked radio system 201 or a decentralized trunked radio system 203. Acentralized trunked radio system 201 uses a dedicated or exclusivechannel, which is often referred to as a control channel, forcommunication between subscriber units and a central controller 205.Other terms that sometimes refer to the central controller 205 includetrunking controller, site controller, resource allocator, channelallocator, controller, and other like terms. The subscriber unitsconstantly monitor the control channel for channel assignmentinstructions. In order to start a group call (i.e., a one-to-many call),a subscriber unit requests that a channel is allocated for its use, andthe central controller 205 transmits instructions telling the subscriberunits in the group to switch to a traffic channel assigned for thatcall. A similar process is followed when a subscriber unit starts anindividual call (i.e., a one-to-one call).

A decentralized trunked radio system 203, however, does not require theuse of an exclusive channel. The intelligence or control function forassignment of a channel to a call remains in the subscriber units. Thus,the decentralized trunked radio system 203 can co-exist withconventional users on the same channels without the use of the controlchannel. When a call is initiated by a subscriber unit, the channelassignment is determined by the logic in subscriber units, not by acontroller. In operation, a subscriber unit scans the channels, finds anidle channel and starts a call on the idle channel. The disadvantage ofthe decentralized trunked radio system 203 is that the scan to find anidle channel significantly increases the access time, which oftenprovides for unacceptably high latency delays during call set up.

BRIEF DESCRIPTION OF THE FIGURES

The accompanying figures, where like reference numerals refer toidentical or functionally similar elements throughout the separate viewsand which together with the detailed description below are incorporatedin and form part of the specification, serve to further illustratevarious embodiments and to explain various principles and advantages allin accordance with the present invention.

FIG. 1 is a prior art block diagram illustrating both a conventionalradio system and a trunked radio system.

FIG. 2 is a prior art block diagram illustrating a centralized trunkedradio system and a decentralized trunked radio system.

FIG. 3 is a flowchart illustrating a process used by a subscriber unitto initiate a call in a radio communications system in accordance withan embodiment of the present disclosure.

FIG. 4 is a flowchart illustrating a process used by a subscriber unitthat is idle and tuned to a channel serving as a rest channel in a radiocommunications system in accordance with an embodiment of the presentdisclosure.

FIG. 5 is a block diagram illustrating a radio communications systemwith enhanced data capability in accordance with an embodiment of thepresent disclosure.

FIG. 6 is a flowchart illustrating a process used for selecting achannel to serve as a rest channel in a radio communications system inaccordance with an embodiment of the present disclosure.

FIG. 7 illustrates a diagram of an example of a status message as usedin providing system status to a subscriber unit in accordance with anembodiment of the present disclosure.

FIG. 8 illustrates a diagram of an example of a link control message asused with a rest channel identifier in accordance with an embodiment ofthe present disclosure.

FIG. 9 illustrates an example of a preamble control signaling block(CSBK) as used with a rest channel identifier in accordance with anembodiment of the present disclosure.

FIG. 10 illustrates an example of a rest channel message as used inaccordance with an embodiment of the present disclosure.

FIG. 11 illustrates an example of a status message as used in accordancewith an embodiment of the present disclosure.

FIG. 12 is a flowchart illustrating a method for informing subscriberunits of at least the channel currently serving as the rest channel inaccordance with an embodiment of the present disclosure.

FIG. 13A/B is a flowchart illustrating a process used by a repeater forending a call in the radio communications system in accordance with thepresent disclosure.

FIG. 14 is a flowchart illustrating a process used by a subscriber unitto determine when the call ends in the radio communications system inaccordance with the present disclosure.

Skilled artisans will appreciate that elements in the figures areillustrated for simplicity and clarity and have not necessarily beendrawn to scale. For example, the dimensions of some of the elements inthe figures may be exaggerated relative to other elements to help toimprove understanding of embodiments of the present disclosure.

DETAILED DESCRIPTION OF THE DISCLOSURE

The present invention discloses, in a two-way radio frequency (RF)communications system having at least one repeater, a plurality ofchannels, and a plurality of subscriber units, a repeater receives amessage that identifies a channel that is selected to serve as a restchannel for the system. The channel that is currently serving as therest channel for the system is monitored by subscriber units that areidle in the system. If the repeater determines that the channel selectedto serve as the rest channel is hosted by the repeater, the repeaterperiodically transmits an identity of the channel as the channelcurrently serving as the rest channel on at least the channel.

The present disclosure also provides for polite interaction between atrunked system's repeaters and co-channel repeaters that may be presentin the geographical vicinity of each other. A co-channel repeateroperates by sharing at least some RF spectrum with the trunked repeateror the data repeater. The co-channel repeater is geographicallypositioned such that transmissions from the trunked repeater or datarepeater and the co-channel repeater are likely to mutually interferewith one another. Typically, regulatory rules require a repeater to stoptransmitting when it is not repeating payload traffic on behalf of itssystem's users. The regulatory rules typically further prohibit asubscriber unit belonging to one system from transmitting when asubscriber unit belonging to a co-channel system is alreadytransmitting. It is these regulatory rules that make a continuouslykeyed control channel of a centralized trunked radio system infeasiblefor system operators that are not able to obtain a license grantingexclusive use of a segment of RF spectrum. Thus, the present disclosureaddresses this critical need.

One aspect of the present disclosure is embodied in a system for trunkedRF resources. The RF resources being shared are sometimes also referredto as channels. A channel in a frequency division multiple access (FDMA)system comprises a frequency, while a channel in a time divisionmultiple access (TDMA) system comprises a frequency and a timeslot, anda channel in a code division multiple access (CDMA) system comprises afrequency and a code. The configuration of the present disclosure offersthe advantages of both a centralized and decentralized trunked radiosystem by providing a call access time faster than a centralized trunkedradio system, but does not require an exclusive control channel. Aprincipal difference between the present disclosure as described hereinand both centralized and decentralized trunked radio systems is that thepresent disclosure does not assign a channel to a call, but ratherassigns a channel to the subscriber units that are idle in the system.Let us now refer to the figures that describe the present disclosure ingreater detail.

FIG. 3 is a flowchart illustrating a process used by a subscriber unitduring initiation of a communication or call in a two-way RFcommunication system having at least one repeater, a plurality ofchannels, and a plurality of subscriber units. In operation, thesubscriber units that are idle (i.e., neither transmitting norreceiving) in the system are monitoring a first channel that iscurrently serving as a rest channel at step 301 until it desires toinitiate a call. The channel currently serving as the rest channel istypically an idle channel, but that may not necessarily be the case,particularly when all channels in the system that are eligible to serveas the rest channel are busy (i.e., being used for communications).Eligibility requirements for a channel to serve as the rest channel aredescribed in more detail below.

The subscriber unit monitoring the channel currently serving as the restchannel may periodically receive the identity of the channel currentlyserving as the rest channel at step 303. Since channels may be sharedwith one or more co-channel users, where co-channel users are entitiesthat share at least some RF spectrum in at least a partially overlappinggeographic area, the repeater hosting the channel currently serving asthe rest channel is not permitted to transmit, i.e., “to be keyed”, forthe sole purpose of continuously broadcasting system information asperformed by a purely centralized trunked radio system. Instead, therepeater that is hosting the channel currently serving as the restchannel periodically transmits a beacon signal that indicates to thesubscriber units the presence and location of the channel currentlyserving as the rest channel. Optionally, other information describingthe status of at least some of the other channels in the system is alsoperiodically transmitted (e.g., status of all channels in the system,status of only the channels in the system that have an active call,etc.). In one embodiment, the repeaters use a backend network forinter-repeater communication to share status information amongst eachother, such as a wired local area network (LAN) connected to eachrepeater, however, the present disclosure is not limited to such aconfiguration. It will be recognized that the repeaters can communicateover the LAN using, for example, User Datagram Protocol over Internetprotocol (UDP/IPv4).

When a subscriber unit desires to initiate a new call at step 305, thesubscriber unit determines whether the channel currently serving as therest channel is idle at step 307. If the channel currently serving asthe rest channel is not idle, the subscriber unit may optionally notifythe user that the system is busy at step 309, and return to step 301 torepeat the process. In the example illustrated in FIG. 3, the subscriberunit is only allowed to transmit its call on the channel currentlyserving as the rest channel if the channel is idle. Thus, if the channelcurrently serving as the rest channel is busy, then the subscriber unitwaits for the channel to become idle, or waits until a new channel isselected to serve as the rest channel that is idle. It should be noted,however, that in other embodiments, the subscriber unit may be allowedto transmit its call even if the channel currently serving as the restchannel is not idle, for example, based on preemptive rights or priorityrankings of the subscriber unit currently transmitting a call and thesubscriber unit desiring to transmit a call.

If, however, the subscriber unit determines that the channel currentlyserving as the rest channel is idle at step 307, the subscriber unittransmits the new call on the channel currently serving as the restchannel at step 311. Thus, assignment of a channel to a call is alwaysimplied or performed before the call is requested. Since the channel isimplied, a subscriber unit does not need to request a channel to start acall as required in the prior art trunked radio system, thus improvingthe access time and eliminating the need for a dedicated controlchannel, as required in a centralized trunked radio system.

Upon the start of the new call, the first channel changes its statusfrom currently serving as the rest channel to a traffic channel, and asecond channel in the system is selected to serve as the rest channelfor the system. Thus, the first channel transitions into a trafficchannel when the new call starts, and the second channel transitionsinto serving as the rest channel for the system to be monitored bysubscriber units that are idle in the system when the new call starts.It is important to note, that in some embodiments, only one channelserves as the rest channel for the system at any given time, however,the present disclosure is not limited to such (i.e., more than onechannel may serve as the rest channel). For ease of explanation, thepresent disclosure assumes that only one channel can serve as the restchannel at any given time, unless otherwise indicated. It is alsoimportant to note that the channel currently serving as the rest channelfor the system may change often (e.g., potentially with the start ofevery new call).

Referring back to FIG. 3, the subscriber unit continues to transmit thenew call on the first channel until the new call is completed. Once thenew call is completed (e.g., after the call hang time (i.e., thepredetermined time period after the call is completed when the repeaterremains in a transmit mode (in a keyed or transmitting state)) andreserves the channel for use by subscriber units participating in thecall that was most recently using the channel), the subscriber unitobtains the identity of the channel currently serving as the restchannel at the time the call is completed at step 313. Optionally, thesubscriber unit may also obtain other information describing the statusof at least some of the other channels in the system, for example,whether a channel is idle or processing a call, and the targetidentifier (e.g., talkgroup identifier or individual subscriber unitidentifiers) that is active on each of the channels in the system thatis processing a call, if applicable. Thus, the subscriber unit may leavethe first channel after the new call is completed and tune to either thesecond channel currently serving as the rest channel, if the subscriberunit becomes idle when the call is completed, at step 315 or tune to achannel where a call of interest is active.

Now let us describe the perspective of the subscriber unit that is idlewhen the first subscriber unit initiates the first call with referenceto FIG. 4. In operation, the subscriber units that are idle in thesystem are monitoring the first channel that is currently serving as therest channel, at step 401, at least until the subscriber unit determinesthat a new call is being initiated on the channel. Each subscriber unitmonitoring the first channel may periodically receive the identity ofthe channel currently serving as the rest channel, as noted above, atstep 403. This ensures that the subscriber units know that they arestill monitoring the channel currently serving as the rest channel forthe system.

When the subscriber unit determines that one of the plurality ofsubscriber units initiated a new call on the channel currently servingas the rest channel at step 405, it further determines whether the newcall is of interest to the subscriber unit at step 407. If thesubscriber unit determines that the new call is not of interest at step407, the subscriber unit ceases monitoring the first channel and tunesto a second channel that was selected to replace the first channel asthe rest channel when the first call started at step 409, and repeatsthe process by monitoring the channel currently serving as the restchannel at step 401, in this instance, the second channel. In oneembodiment, each subscriber unit monitoring the first channel receivesthe identity of the second channel that was selected to serve as therest channel when the new call starts on the first channel. In thisembodiment, the second channel replaces the first channel to serve asthe rest channel when the first channel transitions to a traffic channelat the start of the new call. In other embodiments, however, eachsubscriber unit learns the identity of the second channel serving as therest channel by other means (e.g., detecting a specific synchronizationpattern). Again, once tuned to the second channel, the subscriber unitperiodically receives the identity of the channel currently serving asthe rest channel for the system at step 403, and repeats the processflow. The subscriber units may also receive other information describingthe status of at least some of the channels in the system whilemonitoring the channel currently serving as the rest channel.

If, however, the first call is of interest to the subscriber unit atstep 407, the subscriber unit participates in the first call on thefirst channel at step 411. When the call ends at step 413, thesubscriber unit may receive the identity of the channel currentlyserving as the rest channel at step 415 and may tune to the channelcurrently serving as the rest channel at step 409 if it becomes idleafter the call ends.

In some embodiments, the identity of the channel currently serving asthe rest channel may also be periodically received during the call onthe first channel. Receipt of the identity of the channel currentlyserving as the rest channel received periodically during the call allowsthe subscriber unit to easily tune to the channel currently serving asthe rest channel for the system if it determines to leave the callbefore the call ends (e.g., the call is no longer of interest to thesubscriber unit) at step 417. Thus, if the subscriber unit does not wishto leave the call before it ends, it continues to participate in thecall at step 411. If, however, the subscriber unit does wish to leavethe call before it ends at step 417, it may obtain the identity and tuneto the channel currently serving as the rest channel at steps 415 and409.

Optionally, the subscriber units may also receive other informationdescribing the status of at least one other channel in the system aswell, during the call and/or after the call ends. Such informationallows a subscriber unit the opportunity to leave its current call andeither participate in a different call of interest or higher priority,or become idle and monitor the channel currently serving as the restchannel. For example, the subscriber unit participating in the call onthe first channel may receive, during the call and/or after the callends, notification of a new call that is starting on another channel inthe system, including the channel currently serving as the rest channel.If there is another call of interest on another channel at step 419, thesubscriber unit may tune to the channel with the call of interest atstep 421; otherwise, it may simply become idle and tune to the channelcurrently serving as the rest channel at step 409, or remain on itscurrent call, if still in progress.

Notification of the channel currently serving as the rest channel duringthe first call also allows a subscriber unit to leave the call it iscurrently participating in and initiate a new call. Under this scenario,a subscriber unit participating in a call that desires to transmit acall of its own ceases participation in the current call, tunes to thechannel currently serving as the rest channel, and follows the processflow described above with reference to FIG. 3, starting at step 301.

Let us now describe an optional embodiment for a radio communicationssystem with enhanced data capability. A RF communications system withenhanced data capability operates to move data destined to a centralizeddata application server off of the trunked radio channels and onto a setof channels dedicated to transporting data. This process is advantageousbecause some types of data (e.g., geographic location tracking data) canpresent a very high load to a system that can make it difficult for asubscriber unit to obtain a channel for use with a voice call. Thisoffers a high degree of frustration to the user because voice calls aretypically more time sensitive than data messages. When a user requests avoice call, the typical expectation is that it is serviced nearlyimmediately. When data is required to be transmitted, however, it isoften acceptable to slightly delay the transmission of the data whilethe subscriber unit waits for a channel to become available. The presentdisclosure addresses the time sensitivity of a voice call by optionallydividing the system into two distinct pools of repeaters: trunkedrepeaters and data repeaters. Each pool of repeaters may be sizedequally or differently to provide a desired QoS based on the anticipatedloading of voice traffic and data traffic.

In detail, FIG. 5 illustrates an example of a radio communicationssystem 501 with enhanced data capability in accordance with anembodiment of the present disclosure. The system 501 may comprise atleast one trunked repeater 503 and subscriber units belonging to one ormore talkgroups 505. The system 501 may additionally comprise a dataapplication server 507, at least one trunked channel control station(TCCS) 509, at least one data repeater 511, and at least one revertchannel control station (RCCS) 513. The trunked repeater 503 may operateto handle all voice calls, all control signaling block (CSBK) controlcalls, unit-to-unit data calls, unit-to-group data calls, dataapplication server-to-unit data calls, and data applicationserver-to-group data calls. The data repeater 511 may operate to handleall unit-to-data application server data calls. The data applicationserver 507 may operate as a computer for hosting the data applicationsfor the system 501, such as location tracking, text message, presence,and/or telemetry.

To facilitate reception of data communications from the subscriberunits, SU1-SU6, to the data application server 507, at least one RCCS513 per data repeater channel is installed and connected to the dataapplication server 507. Each RCCS 513 present in the system isconfigured to operate on only its associated data repeater channel.While the primary function of the RCCS 513 is to receive inbound datafrom the subscriber units, SU1-SU6, when confirmed data signaling isused, the RCCS 513 supplies the appropriate open system interconnection(OSI) layer 2 or data link layer response, such as acknowledgement(ACK), negative acknowledgement (NAK), or selective automatic repeatrequest (SARQ), and manages the layer 2 confirmed delivery and packetreassembly processes that are well known in the art. To facilitate thetransmission of data communications from the data application server 507to the subscriber units, SU1-SU6, the TCCS 509, which may operate as atrunked subscriber unit, is installed and connected to the dataapplication server 507. While the primary function of the TCCS 509 is totransmit outbound data to the subscriber units, SU1-SU6, when confirmeddata signaling is used, the TCCS 509 waits to receive the appropriateOSI layer 2 response (e.g., ACK, NAK, or SARQ), and manages the layer 2confirmed delivery and packet fragmentation processes that arewell-known in the art.

It should also be noted, that in some system configurations, datadirected to the data application server 507 may take place using thetrunked repeater 503, in which case the system has no data repeaters511. In this case, at least one RCCS 513 per trunked repeater channel isinstalled and connected to the data application server 507. Each RCCS513 is configured to operate on only its associated trunked repeaterchannel. This configuration is less preferred on systems with a highdata load, but may be acceptable for systems with a lower data loading.

Thus, in operation, in a two-way RF communications system having atleast one repeater, a plurality of channels, and a plurality ofsubscriber units, a subscriber unit monitors a first channel that iscurrently serving as a rest channel for the system. As noted above, thechannel currently serving as the rest channel is monitored by subscriberunits that are idle in the system. When a subscriber unit desires totransmit a call, the subscriber unit determines if the call is aserver-based data call. If not, the subscriber unit follows the processflow described above in FIG. 3, starting at step 307. If, however, thecall is a server-based data call, the subscriber unit selects a secondchannel to transmit the server-based data call from a group of channelsdedicated to handle server-based data calls, and begins transmitting thesever-based data call on the second channel.

Once the server-based data call ends, depending on the system design,the subscriber unit may return to the first channel that was serving asthe rest channel for the system prior to the server-based data call.Alternatively, once the server-based data call ends, the subscriber unitmay receive at least an identity of a third channel currently serving asthe rest channel when the server-based data call ends. Thus, the channelcurrently serving as the rest channel at the time when the server-baseddata call ends may be the same channel that was serving as the restchannel before the subscriber unit tuned to the second channel totransmit its server-based data call, or it may be a different channel.Upon receipt of the identity of the channel currently serving as therest channel, the subscriber unit may leave the second channel and tuneto the channel currently serving as the rest channel, if the subscriberunit becomes idle after the server-based data call ends. If thesubscriber unit receives additional information (e.g., a status messageof at least one other channel in the system) when the server-based datacall ends, the subscriber unit may leave the second channel and tune toa channel where a call of interest is active.

As mentioned above, the channel currently serving as the rest channelmay change often. Let us now describe how a new channel is selected toserve as the rest channel for the system. In order for a new channel tobe selected to serve as the rest channel, all repeaters must first haveknowledge of the state of the other channels in the system. To obtainthis knowledge, when a repeater powers-on, the repeater registers itspresence with a master repeater for the system and may do so using oneof any number of known methods. Until a repeater successfully registerswith the master repeater, the repeater may function as a single repeatern-channel trunked system, where n is the number of channels being hostedby that repeater.

To begin functioning as a single repeater n-channel trunked system, therepeater selects one of its channels to serve as the rest channel forthe system and the status of the other channels on the repeater are setto “idle”. The repeater broadcasts the identity of the channel servingas the rest channel. The broadcast of the identity of the channelserving as the rest channel also informs the subscriber units of thepresence of the repeater. Optionally, the repeater may also broadcastthe status of at least one channel over all of its channels. Thisprocedure is followed by the repeaters that power-on when the masterrepeater is not available (e.g., due to a device, networking, orcommunication link failure) or if the repeater is serving as the masterrepeater, and no other repeaters have yet registered with it.

Upon a successful registration by a repeater with the master repeater,the master repeater may provide the state and UDP/IPv4 addresses of allthe registered repeaters in the system to the repeater, and also mayprovide the state and UDP/IPv4 address of the repeater to all theregistered repeaters in the system. Each of the repeaters in the systemuse the provided information to exchange the status of their channels(e.g., unusable, idle, rest, busy) with each other throughout the courseof operation of the trunked system. In this case, when a channel isbusy, the exchanged information may additionally comprise the type ofcall (e.g., group call, individual call, voice call, data call) or thetarget identifier/call's destination ID (i.e., the identity of the groupor individual who is to receive the call). The exchanged information isused by the repeater in at least two situations, including the selectionof a new channel to serve as the rest channel for the system and theformation of messages which are broadcast to subscriber units informingthem of the state of at least one of the channels in the system.

Let us first discuss the first situation in which the exchangedinformation is used by the repeaters in greater detail: the selection ofa new channel to serve as the rest channel for the system. In oneembodiment, in a two-way RF communication system having at least onerepeater, a plurality of channels, and a plurality of subscriber units,a repeater hosts a first channel that is currently serving as a restchannel for the system. Again, the channel currently serving as the restchannel for the system is being monitored by subscriber units that areidle in the system. At some point, the repeater determines to select anew channel to serve as the rest channel for the system, and makes afurther determination as to whether there is at least one eligiblechannel to serve as the rest channel for the system.

A channel is considered eligible to serve as the rest channel for thesystem when the hardware and/or software for the repeater hosting thechannel is operational, when the hardware and/or software for therepeater hosting the channel is enabled, or when the repeater hostingthe channel does not detect interference. If no interference is detectedand all hardware and/or software is deemed to be in an operable andenabled condition, then other repeaters, if present and in communicationwith the system, are informed that the channels hosted by the repeaterare eligible to serve as the rest channel for the system. Conversely, ifany hardware and/or software failure is detected such that a repeater isin a disabled or inoperable condition or if interference is detected,then the channels hosted by the repeater are deemed ineligible to serveas the rest channel, and all the other repeaters are informed of such.In systems that employ a pool of “data-only” channels (as describedabove), the “data-only” channels are not eligible to serve as the restchannel.

When a channel is deemed to be either in an eligible or ineligiblecondition, the repeaters in the system continually run a series of testsfor determining interference presence and hardware and/or softwareoperability of a repeater in order to determine if a channel remains ineither an eligible or ineligible condition. In one embodiment, sinceeach of the repeaters in the system are interconnected using a backhaulnetwork, such as a LAN connection or the like, all repeaters can beinformed of the status for determining if a specific channel remainseligible or ineligible to serve as the rest channel.

Therefore, if a repeater is inoperable or disabled, all channels hostedby that repeater are taken out of service. In the case of interference,since the interference typically affects all of the channels in thatsection of radio spectrum, such as a physical radio channel, thosechannels in that spectrum affected by the detected interference are alsotaken out of service. If the repeater, however, experiences some type offailure, then all channels being hosted by that repeater are taken outof service. In the case of repeater failure, this one event disables allof the channels hosted by the repeater.

If there is at least one eligible channel in the system, the repeaterselects the new channel to serve as the rest channel from one of theeligible channels in the system. Once the new channel is selected toserve as the rest channel for the system, its identity is broadcasted tothe subscriber units monitoring the first channel. The identity of thenew channel that is selected to serve as the rest channel for the systemmay also be broadcasted to the subscriber units monitoring any of theother channels hosted by the repeater, and to at least one otherrepeater, if other repeaters are present in the system.

It is advantageous for the new channel that is selected to serve as therest channel for the system to be selected from one of the eligiblechannels that is currently idle in the system, however, it is notnecessary. If, however, all the channels that are eligible to serve asthe rest channel are busy, then, in one embodiment, if the channelcurrently serving as the rest channel is eligible, it continues to serveas the rest channel in addition to, for example, transmitting the call.In other words, for example, when a call starts on the first channelcurrently serving as the rest channel, and there are no channels thatare idle in the system that are eligible to serve as the rest channelfor the system, then the repeater does not select a new channel to serveas the rest channel, and the subscriber units that are idle in thesystem continue to monitor the first channel, but do not participate inthe call being transmitted on the first channel. Thus, under theseconditions, the first channel plays dual roles as both the rest channeland a busy traffic channel (i.e., a busy rest channel).

Meanwhile, if a channel becomes idle that is eligible to serve as therest channel, then that channel is selected to serve as the rest channelfor the system. The repeater hosting the busy rest channel informs therepeater that is hosting the eligible channel that just became idle thatit is now hosting the channel that is serving as the rest channel. Therepeater hosting the busy rest channel also informs the subscriber unitsmonitoring the busy rest channel of the channel that is currentlyserving as the rest channel for the system. As a result, the subscriberunits that are not participating in the call may then tune to thechannel currently serving as the rest channel.

There may be various reasons for determining to select a new channel toserve as the rest channel for the system. Some example reasons may be,but not limited to, a new call starting on the channel currently servingas the rest channel, the repeater hosting the channel currently servingas the rest channel has failed, the repeater hosting the channelcurrently serving as the rest channel has become disabled, the repeaterhosting the channel currently serving as the rest channel has detectedinterference, or the like. It should be noted that interference can bedetected on the repeater's uplink channel, downlink channel, or both.When the repeater hosting the channel currently serving as the restchannel fails, becomes disabled, or detects interference, the channel isno longer eligible to serve as the rest channel; moreover, if any ofthese conditions occur, a new channel is selected to serve as the restchannel, however, in the event that the repeater hosting the restchannel fails, the repeater hosting the channel currently serving as therest channel is unable to select the new channel to serve as the restchannel. In this case, at least one of the other repeaters in the systemselects the new rest channel to serve as the rest channel. When therepeater determines that the failure has been corrected, enabled, or nolonger detects interference on the channel, the channel regainseligibility to serve as the rest channel for the system again. If thesystem comprises more than one repeater, the repeater informs at least asecond repeater that its channels are eligible to serve as the restchannel for the system again.

In an alternative embodiment, instead of automatically selecting a newchannel to serve as a rest channel when a new call is initiated, therepeater may determine if the new call will be less than a predeterminedduration. If the new call will be less than the predetermined duration,the repeater does not select a new rest channel to serve as the restchannel, thus allowing the subscriber units that are idle in the systemto remain on the channel during the new call. The repeater determinesthe duration of the call by determining the type of call beingtransmitted, and based on the type of call being transmitted, it caninfer the duration of the call or decode information supplied by thetransmitting device to determine the duration of the call. Some examplesare, but not limited to, (1) when the Data Type field indicates CSBK,the duration of the call is known to be one burst since CSBKs are alwaysonly one burst in accordance with the ETSI-DMR standard; (2) when the“Blocks to Follow” field in the standard ETSI-DMR data header ormultiple block CSBK (MBC) header indicate how many blocks of data willfollow; (3) when the CSBK Blocks to Follow field in the standardETSI-DMR preamble CSBK message indicates how many blocks will follow;(4) when the Data Type field indicates privacy indication (PI) header orvoice link control (LC) header, the subscriber unit assumes a voice callthat is longer than the predetermined duration; or (5) when the DataType field indicates a MBC continuation (without previously receiving aMBC Header), Rate ½ Data (without previously receiving a Data Header),Rate ¾ Data (without previously receiving a Data Header), or Rate 1 Data(without previously receiving a Data Header), the repeater assumes anon-voice call that is longer than the predetermined duration. A personof ordinary skill in the art will readily understand, in view of thepresent disclosure, that the subscriber unit can be provisioned to makeany assumptions regarding the expected duration of a particular type ofcall.

If the new call is assumed to be less than a predetermined duration,then the channel carrying the new call continues to serve as the restchannel. The subscriber units that are idle in the system may continueto monitor the channel; however, a particular subscriber unit will notparticipate in the call if the call is not of interest to the subscriberunit. If, however, the call is not assumed to be less than thepredetermined duration, the repeater attempts to select a new channel toserve as the rest channel at step 409. If the duration of the new callis not known or cannot be determined a priori, the repeater may assumethat the call is not less than the predetermined duration and attempt toselect a new channel to serve as the rest channel for the system. As anexample, the predetermined duration may be set at 500 msec. in somesystems. In another example, the predetermined duration may be set at 0seconds.

Moving on, it is important that a subscriber unit is able to detect whenthe repeater hosting the channel currently serving as the rest channelfails, becomes disabled, or detects interference without notificationfrom the repeater that a new channel has been selected to serve as therest channel. As such, the subscriber unit may need to independentlydetermine that a new channel has been selected to serve as the restchannel.

One way the subscriber unit independently determines that a new channelhas been selected to serve as the rest channel is as follows. Inoperation, the repeater hosting the channel currently serving as therest channel periodically broadcasts a “beacon” message on the channelserving as the rest channel, where the beacon message may simplyidentify the channel currently serving as the rest channel. If asubscriber unit does not receive a predetermined number, n, ofconsecutive beacons on the channel currently serving as the rest channelwithin an expected time period, then it determines that either therepeater has failed, been disabled, or detected interference, and thus,it is no longer monitoring the channel currently serving as the restchannel for the system, or it is not in a coverage area of the repeater.In one embodiment, the beacon period is set to one second and n is setto two. Failing to receive n consecutive beacons triggers the subscriberunit to begin a search for the new channel currently serving as the restchannel using known channel scanning techniques. Each subscriber unitthat is idle in the system, however, may search for the new channelcurrently serving as the rest channel at approximately the same time.Since some search procedures may involve waking up a repeater, thetransmission of subscriber units searching for the channel currentlyserving as the rest channel may collide. To reduce the probability ofcollision, a subscriber unit delays its search by a random time, and allthe repeaters wake up and transmit at least the identity of the channelcurrently serving as the rest channel for a short duration upondetermining that the repeater hosting the channel currently serving asthe rest channel has failed, been disabled or detected interference,thus making the channel currently serving as the rest channel ineligibleto continue serving as the rest channel.

Additionally, there may be instances when the subscriber unit not onlyhas to independently determine that a new channel has been selected toserve as the rest channel, but also has to independently search and findthe channel currently serving as the rest channel. Details of how thesubscriber unit may independently search for and find the new channelcurrently serving as the rest channel is described in co-pending U.S.application Ser. No. ______, titled “Method for EfficientlySynchronizing to a Desired Timeslot in a Time Division Multiple AccessCommunication System”, developed by and assigned to Motorola, Inc.

Using one of the synchronization pattern methods as described inco-pending U.S. application Ser. No. ______, the channel currentlyserving as the rest channel for the system carries a rest channelsynchronization pattern that is mutually exclusive from all othersynchronization patterns being carried on other channels in the system,for example, standard ETSI-DMR synchronization patterns are used onother channels in the system (i.e., channels that are not currentlyserving as the rest channel). It is important to note that when achannel is serving as the rest channel, it carries the rest channelsynchronization pattern; however, when the same channel is no longerserving as the rest channel, it no longer carries the rest channelsynchronization pattern, but rather a synchronization patterns beingcarried by the other channels in the system (i.e., the channels that arenot currently serving as the rest channel). Thus, when the subscriberunit is searching for the channel currently serving as the rest channel,it searches for the rest channel synchronization pattern, which is onlybeing carried by the channel currently serving as the rest channel. Oncethe rest channel synchronization pattern is detected, the subscriberunit immediately synchronizes to the channel since it knows with a highdegree of certainty that it found the channel currently serving as therest channel. Once synchronized, the subscriber unit determines if anyother synchronization patterns are received (e.g., standard ETSI-DMR)which may indicate a new call is starting on the channel or the systemhas selected yet a different channel to serve as the rest channel. Thismethod of identifying a specific channel (i.e., frequency and timeslot)is very quick and useful in conventional channel scanning applications.

FIG. 6 is a flowchart diagram illustrating a process used for selectinga new channel to serve as the rest channel in a two-way RFcommunications system having at least one repeater, a plurality ofchannels and a plurality of subscriber units. The process begins at afirst repeater hosting a channel that is currently serving as the restchannel for the system at step 601. As described above, the channelcurrently serving as the rest channel for the system is monitored bysubscriber units that are idle in the system. At some point, therepeater (typically the repeater hosting the channel currently servingas the rest channel) determines that a new channel needs to be selectedto serve as the rest channel 603. A new channel to serve as the restchannel may be needed if, for example, a new call starts on the channelcurrently serving as the rest channel (i.e., the channel becomes busy),the repeater hosting the channel currently serving as the rest channeldetects interference, the repeater hosting the channel currently servingas the rest channel malfunctions or experiences a failure, or therepeater hosting the channel currently serving as the rest channel isdisabled by the system operator. It is important to note that a busychannel is still eligible to serve as the rest channel for the system,as in the first example. In the latter three examples, however, thechannels hosted by those repeaters become ineligible to serve as therest channel; once the repeater determines that the condition thatcaused the need for a new channel to serve as the rest channel has beencorrected (e.g., interference is no longer present on the channel, thefailure on the repeater has been corrected, or the repeater is enabled),it informs at least a second repeater that its channels are eligible toserve as the rest channel for the system again.

Once the repeater determines to select a new channel to serve as therest channel at step 603, then a further determination is made as towhether there is at least one eligible channel in the system to serve asthe rest channel at step 605. If there are no eligible channels in thesystem, then the process ends without a new channel being selected toserve as the rest channel at step 607. When an eligible channel becomesavailable, it becomes the rest channel for the system.

If there is at least one eligible channel to serve as the rest channelin the system at step 605, the repeater selects the new channel to serveas the rest channel from one of the eligible channels at step 609. Therepeater at least broadcasts the identity of the new channel that willserve as the rest channel to the subscriber units monitoring the channelcurrently serving as the rest channel. The repeater may also broadcastthe identity of the new channel that will serve as the rest channel tothe subscriber units on other channels hosted by the repeater. Therepeater may further broadcast the identity of the new channel that willserve as the rest channel to at least a second repeater, if present inthe system.

It is advantageous, but not necessary, to have a channel that is idle toserve as the rest channel. Thus, in one embodiment, if a repeaterhosting the channel currently serving as the rest channel is alsohosting a second channel that is idle and is eligible to serve as therest channel for the system, the repeater may select the second channelit is hosting to serve as the rest channel before considering selectinga different channel from one of the eligible channels as the new channelto serve as the rest channel. In another embodiment, a repeater mayattempt to select the new channel to serve as the rest channel from oneof the eligible channels that are idle before considering selecting thenew channel to serve as the rest channel from one of the eligiblechannels that are busy. In yet another embodiment, if there is more thanone eligible channel that is idle, a repeater may select the new channelto serve as the rest channel based on, at least in part, an amount ofinterference expected on each of the eligible channels that are idle, ormay select the new channel to serve as the rest channel based on, atleast in part, a ranking assigned to the plurality of eligible channelsthat are idle. For example, the repeaters may have a predefined rankingof channels in order of preference for serving as the rest channel(e.g., a channel with no co-channel repeaters may be the channel mostpreferred to serve as the rest channel while a channel with a highlyactive co-channel repeater may be the channel least preferred to serveas the rest channel).

Even though it is advantageous to select a channel that is idle to serveas the rest channel, it may not always be possible. Thus, if there is atleast one eligible channel in the system, but none of the eligiblechannels are idle, the repeater may select an eligible channel that isbusy as the new channel serving as the rest channel. Eventually, when aneligible channel becomes idle in the system, the repeater hosting thecurrent channel serving as the rest channel that is busy may select theeligible channel that became idle to serve as the rest channel. In oneembodiment, if there are no eligible channels that are idle in thesystem, and if the channel currently serving as the rest channel iseligible, the channel currently serving as the rest channel may continueto serve as the rest channel (i.e., the most preferred busy channel). Inanother embodiment, if there are a plurality of eligible channels in thesystem, but none of the eligible channels are idle, a repeater mayselect the new channel to serve as the rest channel based on, at leastin part, an amount of interference expected and/or measured on each ofthe eligible channels, or may select the new channel to serve as therest channel based on, at least in part, a ranking assigned to each ofthe eligible channels that are busy.

It is also advantageous, but not necessary, for there to only be onechannel serving as the rest channel for the system at any given time.There may be, however, more than one channel serving as the rest channelfor the system, for example, in the following situation. During thecourse of operation of a trunked communication system, the backendnetwork used for inter-repeater communications may be disrupted or fail.In this situation, a first group comprising one or more repeaters maybecome isolated from a second group comprising one or more repeaters,thereby creating a bifurcated network. When this occurs, some subscriberunits may be tuned to a channel currently serving as a rest channel thatis hosted by a repeater in the first group while other subscriber unitsmay be tuned to a channel currently serving as a rest channel that ishosted by a repeater in the second group. One reason why this may occuris if the system, prior to the bifurcation, had only one rest channel,the rest channel is either hosted by a repeater belonging to the firstgroup or the second group, and consequentially, one group of repeatersand subscriber units is left without a rest channel. As a result, thegroup of repeaters that do not have a channel serving as a rest channelselects a channel to serve as the rest channel from amongst the channelsbelonging to their group. Once the channel to serve as the rest channelhas been selected, the system operates as two independent trunkedsystems, each with their own rest channel. A person of ordinary skill inthe art will readily understand that the system can bifurcate into morethan two groups, and still remain within the spirit and scope of thepresent invention.

Depending on how the system is designed, once the backend network isfully operational again, and the system is operating as a single trunkedsystem again, the plurality of channels serving as rest channels thatresulted when the system was bifurcated may remain, or the repeatershosting a channel serving as one of the rest channels may select asingle channel to serve as the rest channel for the system based onknown election techniques. For example, one such method is to pre-assigneach of the channels in the system a unique numeric identifier, and whenmultiple rest channels are detected, select the channel among themultiple rest channels with the lowest valued (or highest value)identifier to serve as the rest channel for the system. It should benoted that any number of methods may be used to elect a single channelto serve as the rest channel for the system from the multiplicity ofchannels serving as a rest channel. Thus, when a repeater determinesthat there is more than one channel serving as the rest channel, in oneembodiment, one of the channels is selected to serve as the rest channelfor the system, and the repeater hosting the other channel serving as arest channel changes its status to a “regular” channel (i.e., not a“rest” channel) that is idle in the system; the repeater furtherbroadcasts the new status of its channel to all of the repeaters in thesystem, and instructs all the subscriber units that are idle and tunedto the channel they believe is the rest channel to tune to the channelthat was selected to serve as the rest channel for the system.

Let us now discuss the second situation in which the exchangedinformation is used by the repeaters in greater detail: the formation ofstatus messages (e.g., a CSBK message) which are broadcast to subscriberunits informing them of the state of at least one of the channels in thesystem. FIG. 7 illustrates an example of a first status message that isbroadcast to the subscriber units while the channel is idle. The statusmessage broadcasted while the channel is idle may comprise the status ofone channel in the system, all the channels in the system, or any numberof channels in between. As noted herein, the status message is used tocommunicate information to subscriber units regarding the current stateof the system. The first status message illustrated in FIG. 7 is anexample of the message transmitted on the channel serving as the restchannel, if idle. Repeaters that are transmitting, but have one or morechannels that are idle, may continuously transmit this status message onthe channels that are idle. The information may include, but is notlimited to, the following: the channel currently serving as the restchannel for the system; the status (e.g., rest, idle, busy, disabled,interference detected, malfunctioned, etc.) of every channel in thesystem, and for every busy channel, the target identifier (e.g.,identity of the talkgroup or individual subscriber unit) using thechannel; the status of the channels having an active call along with thetarget identifier, or the like.

The general format of a CSBK message is defined in the ETSI-DMR standardas comprising twelve octets with the format of the first two octets andlast two octets being defined by the standard. Being defined in theETSI-DMR standard, the first two octets and last two octets will not bediscussed herein. The status message can be transmitted as a singleburst or a multi-burst message. FIG. 7 illustrates an example of atwo-burst status message. To facilitate a variable length statusmessage, the status message may include a first/last field (FL) 701 thatfunctions similar to the link control start/stop (LCSS) informationelement defined in the ETSI-DMR standard. The status message also mayinclude a slot number field 703, which may be used to identify in whichtimeslot the status message is transmitted (applicable to a TDMAsystem). A rest channel ID field 705 identifies which channel in thesystem is currently serving as the rest channel. Further, octet 4includes 1-bit of status information for eight system channels 707,allowing each channel to be identified as being busy or idle. For eachchannel identified as being busy in the eight system channels 707, asubsequent octet is included in the status message to further identifythe identity of the target identifier currently using the channel 709.FIG. 7 shows octets 5-10 containing six channel talkgroup identifiers709, which would be the case if channels 1-6 were busy with talkgroupcalls. If more than six channels are busy with talkgroup calls, thenanother octet of channel information, similar to octet 4, 707, would beintroduced followed again by an appropriate number of target identifierscorresponding to each busy channel.

The second burst, illustrated in FIG. 7, has the same format as thefirst burst. The third octet comprises a FL 71 1, a slot number 713, andthe rest channel ID field 714. Octets 4 and 5, 715, comprises talkgroupidentifiers for channels 7 and 8, 707, which would be the case ifchannels 7 and 8 were busy with talkgroup calls. Octet 6, 717, comprisesanother 1-bit of status information for eight system channels. Octets7-10, 719, comprise four talkgroup identifiers, which would be the caseif channels 9-12 were busy with talkgroup calls. Multiple bursts areconcatenated together until all of the channels in the system have beenreported in the status message. Thus, the status message has anadaptable format that comprises fields for talkgroup identifiers foronly the busy channels which allows the status of the system to becommunicated with a minimal number of bursts. This improves thesignaling efficiency and reduces latency.

FIG. 8 is an example of a full link control (LC) message as used withthe rest channel ID field. The 9-octet LC message format is defined bythe ETSI-DMR standard and is primarily used to identify call type andaddressing. The two full LCs presently defined in the ETSI-DMR standardare group voice channel user LC and unit-to-unit voice channel user LC.According to the ETSI-DMR standard, the full LCs can appear instandalone bursts with the 24-bit CRC as shown or embedded within voicecalls with a 5-bit checksum (not shown). The ETSI-DMR standard definesthe format of the first two octets, which will not be discussed herein.Further, the full LC with rest channel ID shown in FIG. 8 is identicalin format and function to the two full LCs presently defined in theETSI-DMR standard with the following exceptions. The source addressfield is shortened from 24-bits to 16-bits, resulting in the sourceaddress occupying octets 8 and 9. Octet 7 identifies the current restchannel ID. Because of this customization of the LC format, the featureset identifier (FID) is set to identify the message as a message formatproprietary to Motorola, Inc. This LC format is used for voice LCheader, embedded LC, and terminator with LC and is embedded only invoice calls. By including a rest channel ID in the full LC, the systemcan identify the new channel to serve as the rest channel of the systemat the beginning of a new call (e.g., by placing in the voice LC header)and can identify the channel currently serving as the rest channel ofthe system (e.g., by placing in the embedded LC or terminator with LC).Those skilled in the art will recognize that the general format for theLC illustrated in FIG. 8, specifically fields 801, 803, 805, 807 and809, are defined by ETSI-DMR standard.

FIG. 9 is an example of a preamble CSBK that is used in connection withthe rest channel ID for data transmissions. As noted above, the generalformat of the CSBK message is defined by the ETSI-DMR standard. Onespecific type of CSBK presently defined in the ETSI-DMR standard is thepreamble CSBK, which may precede other CSBKs and also may precede anydata transmissions. An example of a preamble CSBK shown in FIG. 9 issimilar in format and function to the preamble CSBK defined in theETSI-DMR standard with the following exceptions. The source addressfield is shortened from 24-bits to 16-bits resulting in the sourceaddress occupying octets 9 and 10. Octet 8 identifies the channel IDcurrently serving as the rest channel. Because of this customization ofthe example preamble CSBK format, the FID is set to identify the messageas a message format proprietary to Motorola, Inc. By including thechannel ID currently serving as the rest channel in the preamble CSBK,the system can identify the new channel serving as the rest channel atthe beginning of a new CSBK or data transmission. Those skilled in theart will further recognize that the general format for the CSBK,specifically fields 901, 903, 905, 907 and 909, are defined by ETSI-DMRstandard.

FIG. 10 illustrates an example of a rest channel message used forinforming the subscriber units of the identity of the channel currentlyserving as the rest channel for the system, while a channel is busy witha call. The rest channel message is similar to the general format of aCACH message, also known as a short link control PDU, as defined in theETSI-DMR standard. The rest channel message may comprise a 4-bit shortlink control opcode (SLCO) in the first octet. A FID identical in formatand function to the FID included in the full LC message may be used inthe second octet. Because the rest channel message is not defined in theETSI-DMR standard, the FID may be set to identify the rest channelmessage as a message format proprietary to Motorola, Inc. The thirdoctet may include bits for an opcode, a reserved field, R, and a restchannel field. Finally, the fourth octet may include bits that arereserved for future use.

FIG. 11 illustrates an example of a second status message used forinforming the subscriber unit of the status of a channel in the system,and the target identifier for an active call, while the channel is busy.Similar to the rest channel message described in FIG. 10, the statusmessage may be similar to the general format of a CACH message asdefined in the ETSI-DMR standard. The status message may comprise a SLCOin the first octet, and a FID, identical in format and function to theFID in the full LC message, in the second octet. Because the statusmessage is not defined in the ETSI-DMR standard, the FID may be set toidentify the status message as a message format proprietary to Motorola,Inc. The third octet may comprise bits for an opcode, reserved field, R,and a channel identification field. Finally, the fourth octet maycomprise the identification of the talkgroup.

As described herein, when a new channel is selected to serve as the restchannel, subscriber units not participating in the call may be informedof the current system channel status in order to locate the new channelserving as the rest channel. Accordingly, the following messages may betransmitted to subscriber units according to the following:

-   -   (1) When the repeater hosting the rest channel is not        transmitting (i.e., asleep), it may periodically wake up and        transmit the first status message 700 on all of its channels;    -   (2) When a subscriber unit does not receive the first status        message 700 for a defined period of time (the beacon interval),        it may attempt to wake the repeater as part of an active site        search to prompt the repeater to transmit the status message        700;    -   (3) Repeaters that are transmitting, but have one or more        channels that are idle, may continuously transmit the first        status message 700 in the channels that are idle;    -   (4) At the end of a call (following call hang time), the        repeater may broadcast the first status message 700;    -   (5) When a repeater has at least one active call, the repeater        may continuously transmits the channel ID for the channel        currently serving as the rest channel in the rest channel        message 1000; when a repeater has at least one active voice        call, the repeater may continuously transmits the channel ID for        the channel currently serving as the rest channel in the        embedded LC message 800 for each voice call;    -   (6) When a channel that is idle becomes the rest channel, the        repeater may broadcast the first status message 700 for a short        duration (for example, 5 seconds);    -   (7) When the repeater wakes up and begins transmitting, it may        begin its transmission with the first status message 700;    -   (8) When a new call is beginning, the new call may be preceded        with a LC message 800 in a voice LC header burst; and    -   (9) When a new CSBK or data call is beginning, the new call may        be preceded with preamble CSBK message 900 in a CSBK burst.

FIG. 12 is a flowchart illustrating a method for informing subscriberunits of the channel currently serving as the rest channel for thesystem. The method begins with the repeater receiving the identity of afirst channel that is selected to serve as the rest channel for thesystem as step 1201. Upon receipt, the repeater determines whether it ishosting the first channel that is selected to serve as the rest channelfor the system at step 1203. If not, the repeater waits to receive a newmessage. If the repeater determines that it is hosting the first channelthat is selected to serve as the rest channel for the system at step1203, the repeater keys up, transmits at least the identity of the firstchannel serving as the rest channel for the system on the first channel,and optionally, on each channel that it hosts at step 1205, and dekeys.

After the transmission at step 1205, the repeater may start a beacontimer at step 1207. It should be noted that the repeater keys up priorto each time it transmits a message, and dekeys after it transmits themessage. In other words, the repeater does not continuously transmitwhen it is not transporting user payload which enables the repeater toco-exist with co-channel repeaters as is typically required forco-channel users.

Once the timer is set, the repeater waits to receive a wake-up requestfrom a subscriber unit at step 1209. The ETSI-DMR standard defines therepeater outbound activation CSBK, also commonly known as a wake-uprequest, and associated procedures for waking a repeater that is asleep.The repeater continues to wait until a wake-up request is received oruntil the beacon timer expires at step 1211, whichever is sooner. If thebeacon timer expires prior to receiving a wake-up request from asubscriber unit, the repeater repeats the process starting at step 1205.If, however, a wake-up request is received prior to the beacon timerexpiring, upon receipt of the wake-up request, the repeater stops thebeacon timer at 1213, keys up, and transmits at least the identity ofthe first channel as the channel currently serving as the rest channelfor the system on at least the first channel at step 1215.

Thereafter, the repeater determines whether the subscriber unit isattempting to initiate a call on the first channel at step 1217. If therepeater determines that the subscriber unit is attempting to initiate acall, the repeater processes the call until it ends at step 1219 and1221. After the call ends, the repeater may transmit an identity of thechannel currently serving as the rest channel for the system on at leastthe first channel. Transmitting the identity of the channel currentlyserving as the rest channel on the first channel allows subscriber unitsparticipating in the call to quickly learn the channel currently servingas the rest channel for the system. In some embodiments, statusinformation of at least one channel in the system may also betransmitted to the subscriber units. For example, providing statusinformation for channels with active calls allows the subscriber unitsto make a determination about which channel to tune to next (e.g., thecurrent channel serving as the rest channel for the system or anotherchannel carrying a call of interest).

If, however, the repeater determines that the subscriber unit is notattempting to initiate a call at step 1217, it dekeys and repeats theprocess starting at step 1207. The subscriber unit may have transmittedthe wake-up request without attempting to initiate a call in situationswhere the subscriber unit needs to confirm it has detected the currentchannel serving as the rest channel.

As previously described, it is understood that it is critical forsubscriber units that are idle to be tuned to the rest channel for thesystem when a subscriber unit initiates a new call. Because a repeaterselects the rest channel and communicates its selection to thesubscriber units and transmission latency is incurred when communicatingthe rest channel selection to the subscriber units, it is possible forthere to be a period of time when some subscriber units believe onechannel is currently serving as the rest channel for the system while,in reality, some repeaters believe a different channel is currentlyserving as the rest channel for the system. As mentioned, this racecondition is caused by signaling latency between the repeater andsubscriber units, which may result in a subscriber unit attempting toinitiate a call on the wrong channel (i.e., a channel that is notcurrently serving as the rest channel).

For example, there is signaling latency between the time when therepeater determines a call has ended (i.e., the end of the call hangtimeperiod) and the time that subscriber units learn that the call has endedso that they may tune to the channel serving as the rest channel or tuneto a different channel that has activity of interest. An example ofsignaling latency occurs with the access type (AT) bit in the ETSI-DMRprotocol, which indicates whether the channel is idle or busy, for achannel which is transmitted only once every 60 msec. Another example ofsignaling latency is that it takes approximately 27.5 msec to transmit asingle CSBK message to a subscriber unit. If the subscriber unitinitiates a transmission during this period of signaling latency, thesubscriber unit may not be transmitting on the correct channel, andconsequently the intended recipients may not be on the channel toreceive the call, resulting in a breakdown in communications. Thus, amethod to end a call that prevents such communication breakdowns isneeded and is now described.

FIG. 13 is a flowchart illustrating the process used by a repeater forending a call session in the radio communications system of the presentdisclosure. A call session comprises one or more transmissions from oneor more subscriber units and at least one hangtime (i.e., the timebeginning from when the repeater indicates that the channel is busyuntil the time when the repeater indicates that the channel is idle).The process starts with the repeater processing a new call session atstep 1301. The repeater is in the process of repeating a subscriberunit's voice bursts and/or data/control bursts from a subscriber unit1303. During this process, an AT bit is set to a value that indicatesthe channel is busy (e.g., set to a logical 1), in accordance with theETSI-DMR standard. The repeater continues to repeat the subscriberunit's voice bursts and/or data/control burst until it determines thatthe subscriber unit has stopped transmitting or has “de-keyed” as step1305. When the repeater detects that the subscriber unit is de-keyed,the repeater transitions the channel into a call hangtime state at step1307, as described in the ETSI-DMR standard. The purpose of the callhangtime state is to indicate to all subscriber units monitoring thechannel which users are permitted to transmit on the channel during acall hangtime period (i.e., during the call hangtime period, the channelmay be reserved for the set of users that was most recently using thechannel). During the call hangtime period, the repeater generates andtransmits at least one terminator with LC message at step 1309, asdescribed in the ETSI-DMR standard. While the repeater is generating theterminator with LC message, the repeater keeps the AT bit set to a valuethat indicates the channel is busy (e.g., set to a logical 1).

When the repeater transitions the channel into the call hangtime stateat step 1307, the repeater initiates a call hangtime timer for thechannel that is set for the duration of the call hangtime period whichindicates the amount of time the channel remains in the call hangtimestate at step 13 11. As described previously, the channel transitionsinto a call hangtime state to reserve the channel for subscriber unitsthat were participating in (i.e., partied to) the transmission mostrecently transmitted on the channel. When the AT bit is set to a valuethat indicates that the channel is busy, and subscriber units areprovisioned with a polite channel access rule, the subscriber units thatare not participating in the call session are not allowed to transmit onthat channel, as described in the ETSI-DMR standard. The channel accessrule is specified during system installation or commissioning by usingradio service software (RSS) or customer provisioning software (CPS)provisioning within the subscriber unit. This includes a subscriber unitthat is monitoring a busy rest channel as described above (i.e. thechannel serves as both the rest channel and a busy traffic channel).Even if the subscriber unit is monitoring the busy rest channel, but isnot participating in the call session, then the subscriber unit is notallowed to transmit when the AT bit is set to a value that indicatesthat the channel is busy.

Conversely, subscriber units that are participating in the call sessionthat is active on the channel may transmit on the channel, even if theAT bit is set to a value that indicates that the channel is busy, as itis reserved for them as indicated by the contents of the terminator withLC message that is sent from the repeater to the subscriber units duringthe call hangtime period (step 1309).

During the call hangtime period, the repeater continually determines ifit received a subscriber transmission at step 1313. If not, the repeaterwaits for the call hangtime timer to expire or “time out” at step 13 15.If the repeater detects a new subscriber unit transmission, then thecall hangtime timer is stopped at step 1317 and the process flowrepeats, starting at step 1303, by processing and repeating burst fromthe new transmission of the subscriber unit that just keyed within theexisting call session.

When the call hangtime timer expires (i.e., at the end of the callhangtime period for the channel) at step 1315, the repeater transitionsthe channel into a system hangtime state at step 1319, which is notdescribed in the ETSI-DMR standard, but is disclosed herein. The purposeof the system hangtime state is to indicate to all subscriber unitsmonitoring the channel the current state of the system so the subscriberunits can make a decision about which channel to monitor at the end of asystem hangtime period, and to reserve the channel as previouslydescribed for the call hangtime state. Subscriber units are not allowedto begin a transmission while the channel is in the system hangtimestate, so in the event of a subscriber unit beginning a transmissionduring the transition from call hangtime to system hangtime, when acritical race condition could occur, the repeater may allow thetransmission to begin, and the participants of the call session arestill on the channel because they have not received a transition fromsystem hangtime to channel hangtime.

During the system hangtime period, the repeater generates and transmitsthe first status message, as illustrated in FIG. 7, and the system keepsthe AT bit set to a value that indicates the channel is busy since thechannel is still reserved for the subscriber units that wereparticipants of the most recent call session at step 1321. Again, thisprevents subscriber units that were not part of the call session fromtransmitting during the system hangtime period.

When the repeater transitions the channel into the system hangtime stateat step 1321, the repeater initiates a system hangtime timer for thatchannel that is set for the duration of the system hangtime period whichindicates the amount of time the channel remains in the state hangtimestate at step 1323. The duration of the system hangtime period istypically short; only long enough to transmit the first status messageto the subscriber unit on the channel. In one example, the systemhangtime period may last only 180 msec. During the system hangtimeperiod, the repeater continually determines if it received a subscribertransmission at step 1325. If not, the repeater waits for the systemhangtime timer to expire or “time out” at step 1327. If the repeaterdetects a new subscriber unit transmission, then the system hangtimetimer is stopped at step 1329 and the process flow repeats by processingand repeating bursts from the transmission of the subscriber unit thatjust keyed.

When the system hangtime timer expires or “times out” at step 1327, therepeater transitions the channel to a channel hangtime state at step1329. The repeater may continue to transmit a status message, asillustrated in FIG. 7, but it sets the AT bit to a value that indicatesthe channel is idle at step 1331. The transition of the AT bit changingfrom a value that indicates that the channel is busy to a value thatindicates that the channel is idle, while receiving at least theidentity of the channel currently serving as the rest channel, andpossibly status information of at least one channel in the system, is anindication that the call session has ended and this triggers thesubscriber units that they may leave the channel and tune to the channelcurrently serving as the rest channel, or possibly, another channelcontaining call activity of interest. Thereafter, the process ends.Thus, during call hangtime period, the repeater identifies which groupis permitted to use the channel, and during the system hangtime period,the repeater identifies the channel which is currently serving as therest channel for the system.

The start of the channel hangtime state is signaled by the transition ofthe AT bit changing from a value that indicates that the channel is busyto a value that indicates that the channel is idle. Those skilled in theart will recognize that the ETSI standard specifies that during thechannel hangtime period, the system or the repeater should broadcast an“idle message.” In contrast to the ETSI standard, the present disclosuretransmits at least the identity of the channel currently serving as therest channel for the system, and optionally, status information for atleast one other channel in the system, during the channel hangtimeperiod so that a subscriber unit can immediately know the channelcurrently serving as the rest channel, and possibly the state of thesystem, and determine which channel it should tune to and operate onnext. If the repeater detects a subscriber unit transmission duringchannel hangtime, the repeater may ignore the transmission by notrepeating it. This is because the subscriber unit should not begin anytransmissions on the channel once the channel has completed itstransition from system hangtime to channel hangtime. By following theabove process for ending a call session, race conditions caused bysignaling delays between the repeater and subscriber units that mayresult in a subscriber unit initiating a call session on the wrongchannel (i.e., a channel that is not the system's current rest channel)are eliminated.

FIG. 14 is a flowchart illustrating the process used by a subscriberunit for ending a call session in the radio communications system of thepresent disclosure. The process starts with a subscriber unit on atraffic channel and participating in a call session at step 1401. Thesubscriber unit checks for an AT bit that is received approximatelyevery 60 milliseconds for its channel. As long as the AT bit is set to avalue that indicates that the channel is busy and the subscriber unit iscurrently participating in the call session on the channel (i.e.,partied to the call), then the subscriber unit is allowed to initiate atransmission since it is on the channel and is currently participatingin the call session on the channel.

Thus, the subscriber unit is continually evaluating whether it needs toinitiate a transmission at step 1405. An example of this is that thesubscriber unit continually determines if a user has pressed a push-totalk (PTT) switch to make a transmission. As long as no transmissionrequest is detected, a determination is made if the AT bit is still setto a value that indicates that the channel is busy 1403 and thesubscriber unit continues to participate in the call session on thechannel.

If the subscriber unit needs to transmit, it begins transmitting on thechannel as long as the AT bit is set to a value that indicates that thechannel is busy at step 1403 and the subscriber unit is currentlyparticipating in call session on the channel at step 1407. If, however,the AT bit is set to a value that indicates that the channel is idle,then the call session is completed and the subscriber unit tunes to adesired channel. In one embodiment, the subscriber unit may receive astatus message similar to one illustrated in FIG. 7 or FIG. 10 at step1409. The desired channel may be the channel currently serving as therest channel, or possibly, a channel that has a call of interest. Hence,when the AT bit transitions to a value that indicates that the channelis idle, the subscriber unit is no longer allowed to transmit on thechannel, leaves the channel that it is currently tuned to and continuesits system operations on a different channel, although in some instancesthe channel currently serving as the rest channel is the same as thechannel that the subscriber unit is currently tuned to, in which case,the subscriber unit continues its system operations on the channel thatit is currently tuned to.

It should be observed that the embodiments reside primarily incombinations of method steps and apparatus components related to a radiocommunications system. Accordingly, the apparatus components and methodsteps have been represented where appropriate by conventional symbolsand flow charts in the drawings, showing only those specific detailsthat are pertinent to understanding the embodiments of the presentinvention so as not to obscure the disclosure with details that will bereadily apparent to those of ordinary skill in the art having thebenefit of the description herein.

In this document, relational terms such as first and second, top andbottom, and the like may be used solely to distinguish one entity oraction from another entity or action without necessarily requiring orimplying any actual such relationship or order between such entities oractions. The terms “comprises,” “comprising,” or any other variationthereof, are intended to cover a non-exclusive inclusion, such that aprocess, method, article, or apparatus that comprises a list of elementsdoes not include only those elements but may include other elements notexpressly listed or inherent to such process, method, article, orapparatus. An element proceeded by “comprises . . . a” does not, withoutmore constraints, preclude the existence of additional identicalelements in the process, method, article, or apparatus that comprisesthe element.

In the foregoing specification, specific embodiments of the presentinvention have been described. However, one of ordinary skill in the artappreciates that various modifications and changes can be made withoutdeparting from the scope of the present invention as set forth in theclaims below. Accordingly, the specification and figures are to beregarded in an illustrative rather than a restrictive sense, and allsuch modifications are intended to be included within the scope ofpresent invention. The benefits, advantages, solutions to problems, andany element(s) that may cause any benefit, advantage, or solution tooccur or become more pronounced are not to be construed as a critical,required, or essential features or elements of any or all the claims.The invention is defined solely by the appended claims including anyamendments made during the pendency of this application and allequivalents of those claims as issued.

1. In a two-way radio frequency (RF) communications system having atleast one repeater, a plurality of channels, and a plurality ofsubscriber units, the method comprising the steps of: at a firstrepeater, receiving a message that identifies a first channel that isselected to serve as a rest channel for the system, wherein a channelthat is currently serving as the rest channel for the system ismonitored by subscriber units that are idle in the system; determiningthat the first channel selected to serve as the rest channel is hostedby the first repeater; and periodically transmitting an identity of thefirst channel as the channel that is currently serving as the restchannel on at least the first channel.
 2. The method of claim 1 furthercomprising the steps of: keying-up the first repeater immediately priorto transmitting; and de-keying the first repeater immediately aftertransmitting.
 3. The method of claim 1 wherein the step of periodicallytransmitting comprises transmitting the identity of the first channel asthe channel that is currently serving as the rest channel in a controlsignaling block.
 4. The method of claim 1 further comprising the stepsof: receiving a wakeup request from a subscriber unit; and transmittingat least the identity of the first channel as the channel that iscurrently serving as the rest channel on at least the first channel. 5.The method of claim 4 further comprising the steps of: determiningwhether the subscriber unit is attempting to initiate a call on thefirst channel; and if the subscriber unit is attempting to initiate thecall on the first channel, processing the call on the first channel;otherwise, dekeying after the first repeater determines that thesubscriber unit is not attempting to initiate the call.
 6. The method ofclaim 5 further comprising, if the subscriber unit is attempting toinitiate the call: selecting a second channel to serve as the restchannel, wherein the second channel replaces the first channel to serveas the rest channel at the start of the call; ceasing the step ofperiodically transmitting the identity of the first channel as thechannel that is currently serving as the rest channel on at least thefirst channel; and transmitting the identity of the second channel asthe channel that is currently serving as the rest channel on at leastthe first channel.
 7. The method of claim 6 further comprisingbroadcasting the identity of the second channel as the channel that iscurrently serving as the rest channel to at least a second repeater. 8.The method of claim 6 further comprising the step of periodicallytransmitting, on at least the first channel, an identity of a channelthat is currently serving as the rest channel, at least until the callends.
 9. The method of claim 8 wherein the identity of the channel thatis serving as the rest channel is transmitted in a common announcementchannel.
 10. The method of claim 9 wherein the identity of the channelthat is currently serving as the rest channel is transmitted in a shortlink control message.
 11. The method of claim 8 wherein the identity ofthe channel that is currently serving as the rest channel is embedded inthe call.
 12. The method of claim 11 wherein the identity of the channelthat is currently serving as the rest channel is embedded in a linkcontrol message.
 13. The method of claim 6 further comprising the stepof, after the call ends, transmitting an identity of a channel that iscurrently serving as the rest channel on the first channel at a timewhen the call ends.
 14. The method of claim 6 further comprising thestep of transmitting on each channel that is idle, and hosted by thefirst repeater, a status message comprising a status of at least onechannel in the system until the call ends.
 15. The method of claim 6wherein the second channel is the same channel as the first channel. 16.The method of claim 1 wherein the step of periodically transmitting isperformed until a second channel is selected to serve as the restchannel.
 17. The method of claim 1 further comprising transmitting, onat least the first channel, a channel status for channels in the systemthat have an active call, and a target identifier for each active call.18. The method of claim 1 further comprising transmitting a statusmessage having a status of at least one channel in the system.